The United States urged Myanmar to address what it called worsened ethnic violence and to accept international monitors to ensure the fairness of closely watched upcoming by-elections.

A US official reiterated that President Barack Obama's administration wanted better ties with the country formerly known as Burma and praised recent moves by the government including the release of hundreds of political prisoners.

"Yet at the same time violence in the Kachin state has worsened with reports of serious human rights abuses and violations of international humanitarian law," said Michael Posner, the assistant secretary of state for human rights.

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Myanmar's nominally civilian government, which took over last year, has reached ceasefires with Shan and Karen rebels in an effort to end ethnic bloodshed that has gripped parts of the country since independence in 1948.